This invention relates to an exposure technique used in pattern transfer in a semiconductor process and, in particular, to a reflective-type mask blank for exposure, a method of producing the same, and a reflective-type mask for exposure.
In the semiconductor industry, a pattern transfer technique is required when an integrated circuit comprising fine or microscopic patterns is formed on an Si substrate or the like. As the pattern transfer technique, use has been made of photolithography utilizing visible light or ultraviolet light. In recent years, accelerated development of a semiconductor device more and more miniaturized and having finer patterns requires a shorter wavelength as an exposure wavelength so as to achieve higher resolution. However, limitation is imposed upon pursuit of a shorter wavelength in existing optical exposure using the above-mentioned photolithography so that the limit of resolution is approaching.
In case of the photolithography, it is known that the limit of resolution in pattern transfer generally corresponds to a half of the exposure wavelength. It is predicted that, even if an F2 laser beam having a wavelength of 157 nm is used, the limit of resolution is on the order of 70 nm. As an exposure technique capable of achieving higher resolution, an EUV (Extreme Ultra Violet) lithography using EUV light is promising. The EUV light has a wavelength shorter than that of the F2 laser beam. It is noted here that the EUV light, which is described in the present specification, is a radiation having a wavelength within a soft X-ray region or a vacuum ultraviolet region, specifically, a wavelength within a range between about 0.2 and 100 nm.
For the EUV light, all substances exhibit high absorption and have a refractive index approximately equal to 1. Consequently, in the EUV lithography, a refraction optical system used in the photolithography can not be used but a reflection optical system is exclusively used. In the reflection optical system, a reflective-type mask is used as a mask (see, for example, a reflective X-ray mask disclosed in Japanese Unexamined Patent Publication No. H08-213303 (JP 8-213303 A)).
The reflective-type mask generally comprises a substrate, a reflective multilayer film formed on the substrate for reflecting light, an intermediate layer formed on the reflective multilayer film, and an absorber film having a predetermined pattern and formed on the intermediate layer to absorb light. Light incident to the reflective-type mask is partially absorbed by the absorber film in an area where the absorber film is present and is partially reflected by the reflective multilayer film in a remaining area where the absorber film is not present. The former area and the latter area may be referred to as an absorption region and a reflection region, respectively. A reflected image formed by the light reflected by the reflective multilayer film is transferred through the reflection optical system onto a wafer. Herein, the intermediate layer serves to protect the reflective multilayer film when the pattern of the absorber film is formed by the use of dry etching or the like in a mask production process. As a material of the intermediate layer, use is often made of SiO2 because SiO2 is excellent in etching selectivity with respect to the absorber film of Cr or the like, readily available, and easy in handling. Generally, in order to increase the reflectivity for exposure light, the intermediate layer formed on the reflection region of the mask (the area where the pattern of the absorber film is not formed) is completely removed after forming the pattern of the absorber film.
However, if the intermediate layer is formed by the material such as SiO2 mentioned above, it is difficult to control an internal stress of the intermediate layer so that a residual stress is produced. As a consequence, the reflective multilayer film is deformed, for example, distorted or warped. This results in degradation in reflection characteristic. In addition, SiO2 has a large surface roughness. Consequently, the absorber film formed thereon has a roughened surface. This results in an increase in edge roughness of the pattern of the absorber film so that the dimensional accuracy of the pattern of the absorber film is decreased.